1) after the first hour both the power transformer AND the choke get hot : it's not so hot I cannot touch but hot enough to strongly warm the hand; is this normal?

2) reading around in the forum George says he would put a B+ of 400V and a 90mA current for the output tubes : but (obviously) I see that if I put up the current then B+ will lower, and reverse lowering the current will raise B+.
The question is : is better go for higher current (say 80 or 90 mA but maybe 350V B+) or thinking that the best operational window for the 300B is around 370-380V?
Or would it be better lower the current to mantain the 300B working around 400V?
Resuming I have no idea of the impact in the hardware and the sound of these choices, so I'm not able to decide what is better.

Boywonder, you are a kind of genius : I'm experimenting arc on startup and up to today I've blown one of FRED (D2) and two Sovtek 5AR4.
I was just wondering what may be.

Maybe I should lower C4 to a SUM of 47 uF?

Talking about 5AR4, I understand that the advantage is that being an indirect heater will give B+ with some dely, so permitting the heating of tube filaments.
Are there alternative tubes that are well experimented?
I blown my two 5AR4 and have to buy more....

Yes, or lower than 47uf. Check the datasheet for whatever 5AR4 that you are using (or thinking about using). They should list a max capacitance value for a given current draw. The further you get below this value, the happier your 5AR4 will be.

The issue with big first caps is that when they are empty on startup they have very low resistance and the current required to put out by the rectifier exceeds it's rating.

The new issue stuff is not as robust as vintage 5AR4's, and 47uf or thereabouts is about the max to try. You should be able to lower C4 down to around 22uf-33uf or less and not impact (reduce) the B+.

I'm actually just a semi-newbie around here like lots of others, although I try to read a lot.

Argh... Another 5AR4 gone
This time the error was that reconnecting all cables I put the red (550V) in place of the yellow (5V-filament) [and the yellow in place od red]

Obviously the tube made a VERY intense light for about three-five seconds.

Change cables to correct configuration and try again : this time the rectifier says nothing until voltage arrives to 200V : at that point it begin to make strange intermittent lights so I decide that has completely gone, throw it away and check situation without any tube.

Instruction say that I should observe some negative voltage on R7, but both on R30 and R7 I see no voltage at all
The transformer is working well.

May have broken some other parts? Is it possible that FRED (D2, D3) have gone? [I don't think] Anyway how check it?
Any more suggestion?